In the U.S. patent application Ser. No. 06/227,081 a method and apparatus in a tracking radar is described for transmitting radar pulses whose carrier frequency varies from one transmitted pulse to the next following, a number M of such pulses forming a series of pulses having the time length 1/f.sub.FRF, where f.sub.FRF is, with respect to the target range R, a chosen quantity. The series of pulses is transmitted sequentially, the number M of carrier frequencies and the order of these being the same from one series to the next following provided that the target range does not exceed a certain value Rk, i.e. R.ltoreq.Rk. If the target range R becomes greater than Rk, a new (and lower) value of the frequency f.sub.FRF is chosen, so that continuously an unambigous determination of the target range can be obtained.
It is previously known in a pulse Doppler radar to suppress clutter, i.e. disturbing echoes from, for example, ground, sea or precipitation by providing so called MTI-filters (Moving Target Indication), see, for example U.S. Pat. No. 3,786,509. Such a filter consists of one or more delay links each with a delay 1/f.sub.PRF, where f.sub.PRF is the pulse repetition frequency of the radar, the signals from the delay links being tapped and added to form an output signal which constitutes the filtered input signal. Such filters show a periodic filter characteristic with a pass band between the so called blind speeds, i.e. for the speeds at which a certain suppression is obtained in addition to the desired suppression of the clutter. The blind speeds then coincide with the speeds whose resulting multiples of Doppler frequency constitute the pulse repetition frequency f.sub.PRF.